A hydrogen filter is known having a gastight chamber in which is provided a coil of tubing of palladium alloy. The coil has at one end an externally accessible input end and at the other end of the coil an externally accessible output end. Impurity-carrying hydrogen is fed under pressure into the input end and a waste gas containing all of the impurities is withdrawn from the output end of the coil. Hydrogen permeates through the palladium-alloy wall of the coil and can be withdrawn from the interior of the chamber. Such a filter is termed an absolute filter as nothing but hydrogen can pass through the walls of the tubing, so that hydrogen of perfect purity can be withdrawn from the chamber.
The tubing typically has a 0.125 inch diameter and a wall thickness of 0.004 inch. Between 5 and 50 feet of such tubing are used in a filter coil. The palladium alloy constituting the tubing is extremely valuable.
In such a filter a mere pinhole leak in the tubing can render the device useless. The extremely valuable palladium-alloy coil must be replaced in its entirety if it develops a leak, as no method has been devised for plugging a leak or replacing a coiled thin-walled tubing section.